Due to pollution caused by fossil fuels, researchers have been seeking ways to harvest energy from green and clean resources. Such energy-harvesting devices include solar cells and hydrogen fuel cells, which harvest energy from various resources and convert the harvested energy into electrical energy.
Moreover, with the recent development of wearable devices and robotics, it is desired that sensors and actuators offer more flexibility. For instance, wearable robotic systems need flexible pressure and temperature sensors to act like artificial skin, and soft actuators to act like artificial muscles. In the human body, skin senses and transmits signals through the nervous system by changing the ion polarity across the cell membranes. Also, muscles can contract based on the sudden change of ion concentration gradient across the cell membrane.
Some piezoelectric materials are known that are capable of generating electrical signals when deformed (that is, pressed or stretched). Examples include some ceramic materials (e.g. lead zirconate titanate (PZT)) and some polymers (e.g. PVDF). Certain other known materials that exhibit the equivalent property when bending or flexing are called flexoelectric materials. An exemplary known flexoelectric material is bent-core liquid crystal elastomers.
One disadvantage of these ceramic materials lies in their difficult synthesis procedures, since they require extreme synthesis conditions, such as high pressure and temperatures. Another drawback is the use of heavy toxic metals for their fabrication. These materials also may need to be embedded when in use, since their weight and rigidity might alter their position during operation.
For PVDF based materials, the manufacturing process is generally tedious and costly. For example, mechanical stretching while applying a high electrical field may be necessary to produce these materials.
Bent-core liquid crystals are special liquid crystals elastomer embedded in elastomer matrices. When deformed, these liquid crystals misalign their dipoles and produce electrical signal when released. These materials possess very low flexoelectric coefficient, and are also difficult to synthesize.